I have USB Vision for my XLR8 Interview device, but discovered it also activates the built-in iSight camera on new G5 iMacs. This way the camera's video can be captured and saved to the hard drive. It also gives you the ability to adjust the video and audio quality, and to record in various resolutions using a wide range of codecs. This is a great application!
One tip: be sure to choose a save location with Snapshot setup and Record setup before using Snapshot or Record, or the application will crash.